A reinforcing bar binding machine which is driven by a motor is provided with a binding wire feeding mechanism for feeding a binding wire to wind it around a reinforcing bar, a binding wire twisting mechanism for grasping and twisting the binding wire wound around the reinforcing bar thereby to strongly tighten the reinforcing bar, and a binding wire cutting mechanism for cutting off a back end of a loop of the binding wire from a following binding wire at the same time when twisting operation of the twisting mechanism starts. In the reinforcing bar binding machine, when an operator pulls a trigger lever, one cycle of operation which includes feeding, cutting and twisting the binding wire is carried out. Structure of this reinforcing bar binding machine is described in many documents, for example, in Patent Documents 1, 2, 3, and 4.
Operation sequence of the reinforcing bar binding machine is controlled by a CPU. When electric current has reached a certain preset value during a twisting step, the CPU judges finish of the twisting step and rotates the motor in a reverse direction to return the twisting mechanism to an initial position, whereby one cycle of the operation is completed.
Patent Document 1: JP-B2-2692495
Patent Document 2: JP-A-2001-038647
Patent Document 3: JP-A-2003-027746
Patent Document 4: JP-A-2003-267307
A battery driven tool, not confined to the reinforcing bar binding machine, has a problem that mechanical performance such as torque, rotation number and so on may be deteriorated due to deterioration of battery performance under low temperature environment. In the reinforcing bar binding machine, finish of the twisting step is judged by detecting an increase of an electric current value during the twisting step. However, because heavy load is applied in the twisting step for twisting a metallic wire, a power supply voltage remarkably drops, and when discharging performance of the battery is deteriorated under the low temperature environment, the power supply voltage instantaneously drops below an operation voltage of the CPU during the twisting step, and the machine sometimes stops during the twisting operation. In this case, the voltage of the battery is recovered by the stop of the operation whereby the CPU is reset, and when the motor is energized, the voltage drops again. By repeating the stop and reset of the CPU in this manner, one cycle of the operation becomes unable to be completed.
Binding strength of the reinforcing bar is proportional to the number of windings of the binding wire loop. In case where high binding strength is required such as in case of constructing a bridge or a road where the binding is conducted by increasing the number of windings of the binding wire to four windings, for example, twisting load is also increased. Therefore, there is such anxiety that stops of operation may frequently occur due to a drop of the voltage under low temperature environment.